1. Field of the Invention
This invention relates to a process for the production and purification of SH-terminated prepolymers obtainable, for example, from a polyether polyol and a thioalkane acid, and to the use of the products obtained in the reaction after their purification.
2. Statement of Related Art
Processes for the production of SH-terminated polyether polyol derivatives have been known for some time from the prior art. Compounds of this type are normally formed by reaction of polyether polyols and bifunctional compounds containing an SH-function, such as for example mercapto-group-containing nitriles, carboxylic acids, carboxylic acid amides, carboxylic acid halides, etc. Products such as these are used, for example, in sealing and coating compositions, in the modification of synthetic or natural fibers, and as auxiliaries in the construction field.
Thus, U.S. Pat. No. 3,881,047 describes SH-terminated polymers and a process for the modification of synthetic fibers free from keratin-containing materials using these polymers which may be obtained, for example, from polyols and thioalkane acids. The polythiols described in U.S. Pat. No. 3,881,047 accumulate during the production process in the form of emulsions which are then directly applied to the fibers and, hence, do not have to be isolated. In addition, by virtue of the reaction by which the polythiols are applied to the fibers after the synthesis step, the degree of reaction does not have to be carefully controlled insofar as only those polythiols which are formed from polyol and thioalkane carboxylic acid become attached to the fibers; this reaction is controlled by the additional components to be added to the reaction mixture. There was no need in the process described in U.S. Pat. No. 3,881,047 to obtain defined products having certain characteristic properties, for example acid numbers below a certain limit.
U.S. Pat. No. 3,549,565 describes a process for the production of elastomeric materials by reaction of polyester polyethers containing mercapto groups and at least one organic dimercapto compound in the presence of oxidizing crosslinking agents, such as for example lead dioxide or zinc oxide. The mercapto-group-containing polyester polyethers to be used in the crosslinking reaction are used in the form of a prepolymer which may be prepared from a polyhydroxypolyether and a mercaptoalkane carboxylic acid. Although the process described in U.S. Pat. No. 3,549,565 attempts--by using the starting materials in stoichiometric quantities--to guarantee a complete reaction insofar as the reaction mixture is not left with any unreacted acidic reactants which could subsequently impair the crosslinking effect of the basic heavy metal oxides, the washing process described in the patent specification, which is also applied in the prior art and which is supposed to guarantee substantial elimination of the excess acid remaining, leads to technical difficulties insofar as emulsions which jeopardize reproducibility of the prepolymer obtained in terms of quality are formed as a product mixture. However, the incomplete removal of the thioalkane carboxylic acid introduced into the reaction as starting material inevitably results in the formation of poorer, unsatisfactory elastomeric materials characterized, for example, by lower hardness or poorer elasticity (depending on the application envisaged for the end products).
Comparable with the process described in U.S. Pat. No. 3,549,565 is the process described by the same patentee in U.S. Pat. No. 3,931,078 for the production of comparable compounds which may also be used in surface coating and sealing compositions. The mercapto groups are again oxidatively crosslinked through the use of basic heavy metal salts or oxides which are highly sensitive to acid residues remaining in the reaction mixture. This process also seeks to achieve complete removal of excess acid, although this cannot be achieved, or cannot be completely achieved, through the proposed process measures of washing out via an emulsion.